Chapter 151 Megaprostheses for Reconstruction Following Tumor Resection About the Knee

METHODS OF FIXATION OF MEGAPROSTHESES

SURGICAL TECHNIQUE

Distal Femoral Replacement Arthroplasty

Proximal Tibial Replacement Arthroplasty

Revision of Failed Hinge Megaprostheses

SUMMARY

Resection of the distal femur or proximal tibia for tumor necessitates reconstruction of the skeletal defect to restore limb function. This chapter will focus on the use of megaprostheses for reconstruction following resection of the distal femur or proximal tibia for the treatment of neoplasm.

Rationale for Use of Megaprostheses

Options for reconstruction following resection of the distal femur or proximal tibia include the use of a megaprosthesis—an implant designed to replace the resected large bone segment—or the combined used of a structural allograft with a revision-type arthroplasty implant (allograft-prosthetic composite). In the past, a perceived advantage of an allograft-prosthetic composite was the potential for bone stock restoration. However, long-term studies have shown that little of the structural allograft actually becomes remodeled to viable bone ⁹ and resorption of the allograft and fracture can occur over time.² Allograft-prosthetic composites provide the potential for effective host tendon healing to allograft tendon, a critical advantage in reconstruction of the extensor mechanism following resection of the proximal tibia (see Chapter 150). Although new implant materials (e.g., trabecular metal) show great potential for effective healing of host tissue and tendon to the actual metal prosthesis, such implants have been designed and should be released to the orthopedic market in the near-future.²⁷

Following resection of the distal femur for tumor, an allograft-prosthetic composite has no clear advantage over a megaprosthesis. In this setting, collateral and cruciate ligament function are best reconstructed by a constrained arthroplasty articulation. Furthermore, a megaprosthesis is appropriate for patients who will receive chemotherapy or radiation therapy after surgery, modalities that could increase the risk of allograft infection and nonunion.

Indications

A segmental modular rotating hinge megaprosthesis knee system is appropriate for reconstruction of the distal femur following tumor resection. Segmental modular systems allow for intraoperative determination of the implant size based on the extent of bone resection necessary for an appropriate oncologic margin. A rotating hinge design is favored over a fixed hinge, which permits motion only in flexion-extension. A rotating hinge design allows motion in flexion-extension, rotation, and longitudinally along the axis of the extremity by permitting some distraction of an inner bearing component with its outer articulation. Compared with a fixed hinged design, such features will minimize stress transfer to the fixation interface and decrease the risk of aseptic loosening.

Following resection of the proximal tibia, a segmental modular rotating hinge megaprosthesis knee system is appropriate for patients who require postoperative radiation or have an extensor mechanism that cannot be reconstructed with an allograft-prosthetic composite (Fig. 151-1). Although chemotherapy can retard bone healing at the allograft-host bone junction, the administration of postoperative chemotherapy is not an absolute contraindication to the use of an allograft-prosthetic composite in the proximal tibia. If trabecular metal pads in the tibial tubercle region of a proximal tibial replacement megaprostheses prove effective in providing a solid method of attachment of the patellar tendon to the implant, I believe that the use of allograft-prosthetic composites for reconstruction of the proximal tibia will significantly diminish.

Figure 151-1 A, Proximal tibial replacement knee arthroplasty following sarcoma resection, anteroposterior (AP) view. Suture holes were placed in the proximal tibial body to assist with extension mechanism reconstruction. B, Lateral view.

In skeletally immature patients, expandable implants will compensate for the loss of growth from resection of the involved physis. Noninvasive means of expansion have been developed that simplify the use of these implants.

Methods of Fixation of Megaprostheses

Fixation of the intramedullary stems of the femoral and tibial components may be achieved with various methods. Both cemented and uncemented fixation have been shown to be effective.^5,^11,^12,^15,²⁸ Cement fixation should be considered for patients with poor-quality bone and those who will require postoperative radiotherapy. Some surgeons favor the use of a hydroxyapatite coating on the intramedullary stem component of megaprostheses to promote fixation of the uncemented stem.^4,^21,^25,²⁶

With press-fit or cement stems, implant systems will typically have an option of a porous-coated collar on the intramedullary stem component (Fig. 151-2). This design feature is to encourage the formation of extracortical bone bridging; this is bone that forms between the host bone and this collared portion of the implant (Fig. 151-3). Bone graft material is placed in this region prior to wound closure to promote bone bridging. Ward and colleagues³⁶ have postulated that extracortical bridging between the host bone and implant with bone or soft tissue may retard osteolysis by preventing implant debris–containing synovial fluid from contacting the bone-implant or bone-cement interface (Fig. 151-4).

Figure 151-2 Intraoperative photograph of modular distal femoral replacement knee arthroplasty. The femoral component is composed of a distal articulating body, an intercalary segment to provide appropriate length, and a proximal intramedullary stem with a porous coated collar. Bone graft has not yet been applied to the junction of the host bone and the porous-coated collar of the implant. The patella was not resurfaced.

Figure 151-3 Close-up radiograph of host bone implant junction illustrating extracortical bone bridging.

Figure 151-4 Distal femoral replacement knee arthroplasty following sarcoma resection. A, AP view showing all-polyethylene tibial components. The metal inner bearing component allows some longitudinal distraction between the inner bearing and cemented tibial component. Polyethylene bushings, an axle, and a polyethylene extension bumper comprise the remainder of the articular portion. In this case, the femoral component consists only of the articular body component and the stem. Bone graft has been placed to promote extracortical bone bridging. B, Lateral view. The patella has been resurfaced on this patient.

The degree of development of extracortical bone bridging has been variable. In the distal femur, extracortical bridging develops more commonly posteriorly and along the compression side of the femur.^18,²⁰ Chao and associates⁶ have found that the total percentage of the length of the porous-coated region that was covered with bone formation in the distal femur and knee was 75% ± 31% in 15 implants, with follow-up of 2 to 21 years. The formation of this extracortical bone bridging stabilizes by 2 years following surgery. They also found that the prevalence of stem loosening is very low, suggesting that extracortical bone bridging improves long-term fixation. Kawai and coworkers¹⁹ have noted an association between extracortical bone bridging and higher limb function scores.

Clinical data have suggested that the presence of extracortical bone bridging is helpful to long-term success. However, the bone callus of the extracortical bone bridging may not actually grow into the porous coating of the implant. Lucent lines between the extracortical bone and porous surface may be observed.¹⁸ Furthermore, histologic analysis of five retrieved tumor megaprostheses has shown no bone ingrowth into the porous-coated segment of the implant, despite radiographs of these implants suggesting bone ingrowth into the porous-coated segment.³⁰ Rather, transmitted light microscopy has shown fibrous tissue between the extracortical bone and porous coating. Nonetheless, the presence of this extracortical bone–fibrous tissue may increase prosthetic stability. With the application of new implant materials that promote the ingrowth of bone more effectively (e.g., trabecular metal), true extracortical bone bridging of bone ingrowth into the porous surface may occur (Fig. 151-5).